1. Field of the Invention
The present invention relates to the art of switching power supplies, and more particularly to a highly efficient switching power supply.
2. Description of the Related Art
Switching power supplies include forward switching power supplies, multiple-transistor switching power supplies, RCC switching power supplies, etc. One type of these switching power supplies has a secondary rectifying circuit for converting AC electric energy generated by a transformer into DC electric energy by way of full-wave rectification.
A conventional multiple-transistor push-pull switching power supply of the full-wave rectification type will be described below with reference to FIGS. 1a and 1b.
FIGS. 1a and 1b show the conventional multiple-transistor push-pull switching power supply, generally denoted by 202, which comprises a transformer 207, a primary switching circuit 215, and a secondary rectifying circuit 225.
The transformer 207 is of a central-type structure and comprises first and second primary windings 231.sub.1, 231.sub.2 and first and second secondary windings 232.sub.1, 232.sub.2 which are magnetically coupled to the first and second primary windings 231.sub.1, 231.sub.2.
The primary switching circuit 215 comprises a control circuit 210 and first and second primary transistors 211.sub.1, 211.sub.2.
Each of the first and second primary transistors 211.sub.1, 211.sub.2 comprises an n-channel MOS transistor. The control circuit 210 is connected to the first and second primary transistors 211.sub.1, 211.sub.2, for applying voltages individually to the gate terminals of the first and second primary transistors 211.sub.1, 211.sub.2.
The first and second primary transistors 211.sub.1, 211.sub.2 have respective source terminals connected to each other and respective drain terminals connected respectively to ends of the first and second primary windings 231.sub.1, 231.sub.2 whose other ends are connected to each other.
A primary rectifying and smoothing circuit (not shown) is connected to a primary side of the switching power supply 202. The primary rectifying and smoothing circuit keeps the source terminals of the first and second primary transistors 211.sub.1, 211.sub.2 at a ground potential, and applies an input voltage V.sub.IN to the junction between the first and second primary windings 231.sub.1, 231.sub.2. When the control circuit 210 turns on the first primary transistor 211.sub.1, a current flows through the first primary winding 231.sub.1, and when the control circuit 210 turns on the second primary transistor 211.sub.2, a current flows through the second primary winding 231.sub.2.
The secondary rectifying circuit 225 comprises first and second diodes 222.sub.1, 222.sub.2. The first and second diodes 222.sub.1, 222.sub.2 have respective anodes connected to respective ends of the first and second secondary windings 232.sub.1, 232.sub.2 whose other ends are connected to each other. The first and second diodes 222.sub.1, 222.sub.2 have respective cathodes connected to each other.
A secondary smoothing circuit comprising a choke coil 223 and a smoothing capacitor 224 is connected to a secondary side of the switching power supply 202. The choke coil 223 has an end connected to the interconnected cathodes of the first and second diodes 222.sub.1, 222.sub.2 and an opposite end connected to an end of the smoothing capacitor 224.
The smoothing capacitor 224 has an opposite end connected to the junction between the first and second secondary windings 232.sub.1, 232.sub.2. The ends of the smoothing capacitor 224 serve as an output terminal 228 and a ground terminal 229.
When the first primary transistor 211.sub.1 is turned on and the second primary transistor 211.sub.2 is turned off, a current flows along a path 241 in the primary side of the switching power supply 202.
At this time, the first diodes 222.sub.1 is forward-biased and the second diode 222.sub.2 is reverse-biased by a voltage induced across the first and second secondary windings 232.sub.1, 232.sub.2. Therefore, the voltage is induced across the first secondary windings 232.sub.1 by magnetic energy transferred from the first primary windings 231.sub.1 to the first secondary windings 232.sub.1. A current flows through the first diode 222.sub.1 into the choke coil 223 along a path 251 in the secondary side of the switching power supply 202, charging the capacitor 224. As a result, a DC output voltage V.sub.OUT appears between the output terminal 228 and the ground terminal 229.
FIG. 1b shows how the switching power supply 202 operates when the first primary transistor 211.sub.1 is turned off and the second primary transistor 211.sub.2 is turned on. In FIG. 1b, a current flows through the second primary windings 231.sub.2 and the second primary transistors 211.sub.2 along a path 242 in the primary side of the switching power supply 202.
When the current flows through the second primary windings 231.sub.2, the first diodes 222.sub.1 is reverse-biased and the second diode 222.sub.2 is forward-biased by a voltage induced across the second secondary winding 232.sub.2. As a result, a current into the choke coil 223 along a path 252 in the secondary side of the switching power supply 202, charging the capacitor 224.
When the first and second primary transistors 211.sub.1, 211.sub.2 of the primary switching circuit 215 are alternately energized, currents flow alternately in the first and second diodes 222.sub.1, 222.sub.2 of the secondary rectifying circuit 225, and are smoothed by the choke coil 223 and the smoothing capacitor 224, thereby producing the DC output voltage V.sub.OUT between the output terminal 228 and the ground terminal 229.
In recent years, there has been a demand for making the switching power supply 202 more efficient. In order to meet such demands, attempts have been made to employ Schottky diodes as the first and second diodes 222.sub.1, 222.sub.2.
However, since the Schottky diodes have low forward conduction voltages, though the loss is small, their reverse leakage current increases as their temperature increases. Because the temperature of the Schottky diodes further rises due to such a reverse leakage current, the Schottky diodes tend to cause thermal runaway.
If the switching power supply 202 is applied to resonant power supplies which have been developed in recent years, then a sine-wave current is supplied to the first and second primary transistors 211.sub.1, 211.sub.2 to thereby to reduce a loss in the primary side. As later described, MOS transistors are used as the first and second diodes 222.sub.1, 222.sub.2, and operated in a third quadrant to thereby rectify a current induced in the secondary windings so as to reduce a loss in the secondary side.
FIG. 2a of the accompanying drawings show the waveform 271 of a voltage applied to the gate terminal of the first primary transistor 211.sub.1 (or the second primary transistor 211.sub.2), and the waveform 272 of a current flowing through the first diode 222.sub.1 (or the second diode 222.sub.2) of the secondary rectifying circuit 225.
In a resonant power supply, gate voltages are also alternately applied to the first and second primary transistors 211.sub.1, 211.sub.2 to render them conductive alternately. During dead times D.sub.T, however, no gate voltage is applied to either one of the first and second primary transistors 211.sub.1, 211.sub.2, so that their conduction periods will not overlap each other.
The voltage and current waveforms shown in FIG. 2a are produced when the switching power supply 202 is under a light load. The secondary diode 222.sub.1, which comprises a MOS transistor, passes the current 272 to rectify the voltage induced across the secondary winding 232.sub.1, and thereafter operates as a transistor to pass a current 273 in a reverse direction. The reverse current 273 flows while the primary transistor 211.sub.1 is being rendered conductive when the switching power supply 202 is under a light load.
When the switching power supply 202 is under a heavy load as shown in FIG. 2b of the accompanying drawings, a reverse current 276 flows in the secondary side during a period T while the primary transistor 211.sub.1 is being turned off with no gate voltage 275 applied thereto. Since the reverse current 276 flowing in the secondary side during the period T also flows into the primary side that is coupled to the secondary side by the transformer 207, the reverse current 276 cancels a partial resonant current (zero-voltage switching current) flowing in the primary side.
As a result, the first and second primary transistors 211.sub.1, 211.sub.2 operate out of a resonant state, thereby resulting in an increased loss.